JPH11-230931A discloses an air-fuel ratio sensing device including a resistor used to sense a current and an air-fuel ratio sensor. In this case, the resistor and the air-fuel ratio sensor constitute a series circuit. The air-fuel ratio sensing device further includes an A/D converter and a microcomputer. A voltage between two ends of the resistor is A/D converted by the A/D converter. The microcomputer senses an air-fuel ratio based on a converting result of the A/D converter.